This invention relates to end-to-end connection of optical fibers and, more particularly, to a variable attenuator for use in such connections.
Fiber optics has become, in most cases, the preferred mode of signal transmission, especially where the signals are in the higher frequencies. Optical fibers produce greatly increased bandwidth over conventional electrical conductors, and are relatively immune to ambient conditions that can disrupt electrical signal transmission. The numerous advantages of optical fibers come with a price, however. Whereas with electrical conductors, splicing may be had by simply butt welding, soldering, or otherwise joining the ends of the conductors being spliced, such is not presently possible with optical fibers, which have a diameter of, for example 125 microns and a core diameter of from 6 to 30 microns, and, in a satisfactory splice, must permit light transmission with a minimum of insertion loss while providing a stable junction. As a consequence, there has been a constant and ongoing effort to achieve, in a connector splice arrangement, an alignment of the butting ends of the fibers to minimize insertion loss. In addition to alignment, the width of the gap between the fiber ends and the surface condition of the ends are factors which must be considered in reducing or minimizing loss. The prior art arrangements are, for the most part, dependent upon or directed toward the centering of the fiber cores, and in most instances, the fibers are contained in ferrules which have centered fiber containing bores therein. On the other hand, in U.S. Pat. No. 4,544,234 of DeVeau et al., the fibers are contained in a slotted tube and centered prior to their being cemented in place within the slot. Such an arrangement does not require centering of the bores of the ferrules, but it does not permit relative movement between the fibers for optimum transmission after the fibers are cemented in place.
In U.S. Pat. No. 4,691,986 of Aberson et al., the disclosure of which is incorporated herein by reference, there is shown an arrangement wherein alignment of the fiber containing bores of plugs or ferrules is achieved, thereby obviating at least to a large extent alignment of the fiber cores. The invention of that patent involves the use of xe2x80x9ccontiguousxe2x80x9d plugs derived from contiguous segments of tubular stock, with the contiguous ends being the mating ends, with the rotational relationship between the plugs being that existed prior to cutting or sectioning of the tubular stock. With such an arrangement, the fiber containing bores are automatically aligned. It doesn""t matter if the bores are slightly eccentric relative to the outer diameter of the plugs, they will still be aligned, thereby enabling alignment of the fiber cores. The invention of that patent is usable in almost any of the large variety of prior art connectors wherein the fibers are contained within ferrules or plugs.
The foregoing is directed to the general problems inherent in most fiber connectors of achieving proper fiber alignment in connectors or splices. These problems also exist in various optical fiber components, such as, for example, variable attentuators. In U.S. Pat. No. 4,986,627 of Boscher et al., there is shown a variable attenuator arrangement that has abutting ferrules having substantially identical outer diameters, but with fiber containing bores which are eccentric with respect to the outer diameters. The bores, each of which has a diameter substantially the same as the outer diameter of the fiber contained therein, are rotatable relative to each other to vary the alignment of the fibers relative to each other and thereby to vary the attenuation by increasing or decreasing the offset between the fiber cores. Such an arrangement requires, for optimum performance, that, at least one rotational position, the ferrule bores are substantially aligned, thereby aligning the fiber cores at least to the extent possible with whatever eccentricity may exist for the fiber cores themselves.
In addition to arrangements such as shown in Boscher et al, other prior art variable attenuators make use of air gaps and/or angled ferrule (and fiber) ends. Ideally, a variable attenuator should provide the needed or desired attenuation, should have low return loss, and should have stable performance in high power systems.
The present invention, hereinafter shown as embodied in a variable attenuator, overcomes or obviates many of the shortcomings of the prior art, as enumerated hereinbefore, while providing reliable desired variations in the attenuation of signals passing therethrough. It should be recognized that in many instances some level of attenuation may be desirable in order to achieve a power balance among several related transmission lines rather than simply a maximum signal power throughput.
The variable attenuator embodying the principles of the invention comprises a fiber connector having a ferrule or glass capillary mounted in a stationary holder and a butting ferrule mounted in a rotatable holder. Each ferrule has a longitudinal bore extending therethrough, and the bores are axially aligned, such as in the manner of the ferrules or plugs shown in the Aberson et al. patent.
Each ferrule bore contains an optical fiber which, in accordance with a feature of the invention, has a diameter that is less than the inside diameter of the ferrule bore to the extent that, when the fibers are in place within the ferrules, rotation of the one ferrule will decrease or increase the degree of engagement of the two fiber ends and of the fiber cores, thereby increasing or decreasing the amount of attenuation therebetween. Initially, the fibers, the ends of which are ground and polished, are aligned such as in the manner disclosed in the DeVeau et al. patent to produce maximum transmission and cemented in place to the respective inner walls of the ferrule bores. Because the diameter of each ferrule bore is greater than the outer diameter of the fiber, the fibers are not co-axial with the bores containing them, and relative rotation of the ferrules produces relative displacement of the fiber ends, thus attenuating the signal being transmitted. The diameter of each ferrule bore is such that the fiber cores contained therein can be transversely separated by approximately the diameter of the fiber core, providing attenuation levels of from 0 dB to approximately xe2x88x9240 dB while maintaining low return loss.